Infrared (IR) communications are widely used in the remote control of consumer electronics devices, such as computers, televisions, and stereo equipment. Binary codes, corresponding to operating commands, are transmitted as modulated IR signals from a remote controller to a target device. The target device demodulates and decodes the signals in order to determine the operation to perform.
Several methods for modulating IR signals are prevalent for consumer electronics infrared (CEIR) transmission. These methods include the RC-5 and RC-6 protocols developed by Royal Philips Electronics, and the NEC protocol, developed by Nippon Electric Corporation. These protocols define how binary codes are encoded as spaces and marks, spaces being periods when no light is emitted by the transmitter and marks being periods when the IR light is pulsed on and off at a particular carrier frequency. (Carrier frequencies between 30 kHz and 56 kHz are typical for CEIR transmission.) Demodulation extracts the envelope of the pulse train corresponding to each mark. Ideally, the envelope of each mark has the form of a single square wave.
Demodulation chips are available for integration into consumer electronics products. Examples of such chips include the “TSOP321 . . . ” series, produced by Vishay Semiconductor (Heilbronn, Germany) and described in specifications available at www.vishay.com/doc?82229. The “TSOP321 . . . ” series is provided in a range of models, each designed for a specific carrier frequency.
Other manufacturers of IR demodulator chips include Standard Microsystems Corporation (“SMSC”) (Hauppauge, New York) and ITE Tech, Inc. (Hsinchu, Taiwan). The IR demodulator chips produced by these manufacturers are generally capable of demodulating IR signals over a range of frequencies. The chips include built-in microcontrollers, which digitally sample received IR signals in order to demodulate the IR signal.
Various methods for demodulating IR signals are described in the patent literature. For example, U.S. Pat. No. 5,898,513, whose disclosure is incorporated herein by reference, describes a method for detecting a carrier frequency by digitally sampling the received IR signal. U.S. Pat. No. 6,407,840, whose disclosure is incorporated herein by reference, describes a method for reducing the energy consumption of an IR demodulator chip by triggering the chip to operate when an IR signal with the expected carrier frequency is detected. U.S. Patent Application Publication 2004/0208182, whose disclosure is incorporated herein by reference, describes a computer configured to receive an IR signal and to execute software to demodulate the received signal.
A retriggerable monostable multivibrator, also referred to as a retriggerable one-shot, may be used to convert a train of digital pulses into a constant digital level. An example of such a device is the DM74LS122 device available from National Semiconductor. The range of carrier frequencies supported by the retriggerable one-shot is set by a combination of an external resistor and capacitor, which also determine a stretch time by which the envelope extends past the last pulse of the pulse train.